FIG. 1 is a diagram illustrating a description of basic operation for a motor driving. The motor driving state is controlled by control of a duty ratio from switching between FET or other switches S1 through S4, which includes this kind of an H bridge circuit. For example, when a large voltage Vout by a counter-electromotive force Vmotor generated by motor rotations is supplied from a battery batt, the motor is in a power running state. When controlling the duty ratio of the switches with PWM (Pulse Width Modulation), and for example the period when the switches S1 through S4 are on is lengthened, then when Vout=Vbatt*Duty (on ratio)>Vmotor, this achieves the power running state (state where torque output is present) for example. When in a power running state, current flows to the motor side, as indicated by the solid arrow in FIG. 1. Also, by adjusting the period when the switches S1 through S4 are on, it is possible to create a state where Vout=Vmotor (torque zero state). Further, by shortening the period when the switches S1 and S4 are on, a state in which Vout<Vmotor is created, and thus there a transition to a battery regeneration state where current flows to the battery batt as illustrated by the dotted arrow in FIG. 1 if the battery batt is a storage battery. Additionally, by adjusting the switching duty ratio of switches S1 through S4, other braking states may be enabled such as a loss brake state that discards the counter-electromotive force Vmotor from the motor.
Further, just as electric power is supplied to the motor in a normal rotation from the switches S1 and S2; of course this may be driven by electric power supplied to the motor from the switches S3 and S4 in an inversed direction.
Technologies to drive motors with a battery such as (1) a diode driving method and (2) a current constant feedback method are well known.
(1) Diode Driving Method
This technology uses a parasitic FET diode or a specialized diode in an H bridge circuit such as illustrated in FIG. 1, and supplies a PWM duty ratio to only the power running direction or the braking direction to apply torque by a broad feed forward. There is no concern for open loop problems, and is well known for its ability to readily and reliably control the power running or the braking direction.
However, efficiency is sacrificed due the drop voltage of the diode. Also, the rectifying action of the diode causes current to flow only in the instant the applied voltage is more than the counter-electromotive force for when power running, and to flow only in the instant the applied voltage is less than the counter-electromotive force for the braking periods. Because the counter-electromotive force is in proportion to the sinusoidal voltage output when compared to the motor rotation speed, the applied voltage is not proportional to the current and therefore also the torque, this causes a problem in which the linearity of torque control greatly worsens, and this linearity is also largely affected by the speed. For this reason, in order to obtain the target torque, an extremely complicated correction control regarding the speed and target torque is necessary.
(2) Current Constant Feedback Method
This method detects motor current in real time, and control the motor current depending on the desired torque supplying a constant feedback regardless of speed at that time.
Although accuracy is high due to a control that monitors the current, which is the control result, the disadvantage is that negative return control can easily cause instability, and so to create a stable feedback control, a control which is a sufficiently low cut-off frequency is requested regarding a loop response (motor current detection→microcomputer calculation→output instruction), which slows the response making it necessary to further increase the speed and accuracy of the loop response. Also, if there is a break up in the information indicating where the return should occur, this could cause a problem where the control amount is determined to be insufficient, and so excessive response attempts are made.
Further, Japanese Unexamined Patent Application Publication No. 10-59262 proposes a method to perform feedback through a proportional circuit and an integrator to determine the deviation between an assist torque instruction (input) and the motor torque (output). Generally, this is called PI control or phase lag correction.
In the case of this control system, the integrator causes phase lag in the high-frequency region, and so there is a possibility that a lag element in the motor, which is the control object, causes oscillation. For this reason, differential circuit is often added to advance the phase at this high-frequency region. However, when complicating the control system in this way, it is necessary to model the inverter and motor that are the control subjects, which may cause a change in properties due to modeling errors or variance and degradation of devices developed from models without errors and therefore possibly cause oscillation.